1. Field of the Invention (Technical Field)
The present invention relates to glow discharge switching apparatuses and methods for high power applications.
2. Background Art
Switching is a major challenge for current and emerging military and commercial applications requiring both high switching speed and high power, such as:
Pulsed Lasers (including CO.sub.2, excimer, and copper vapor lasers); PA1 Electron-beam (E-beam) accelerators and X-ray machines; PA1 Radar (including airborne, ship/ground-based, weather radars, and airport approach control radars); PA1 Electric Guns; PA1 Speed controls for high-power electric motors; and PA1 Controls for high-electrical-power industrial processes featuring repetitive operation (such as assembly-line welding).
Switch requirements for such uses include high voltage and high current handling capability; robust design and high-temperature capability; stable operation for repetitive switch operation; long lifetime; and low maintenance. In general, future switching technology oriented to the above applications must handle voltage levels in the kV to hundreds of kV range, with amperage levels from tens of kA to mega-amperes, along with repetition rates up to 10 kilohertz (kHz). Switches must also offer low timing jitter, low switching delay times, low power loss and elevated-temperature operation.
At present, hydrogen thyratron tubes are the power switch most widely used for the applications noted above. While thyratrons are superior to competing switch designs like mechanical relay switches, solid-state switches and spark gaps, a significant drawback for thyratrons is their need for electrically heated cathodes for producing controlled emissions of cathode electrons. Control of these grids requires sensitive controls; also, grid design compromises are needed to accommodate conflicting electrical, thermal and mechanical requirements.
As a result, thyratrons are costly to produce. They also are difficult to scale up to higher powers. The most serious limitation to thyratrons is the relatively low peak current capability (10 kA typically) and the relatively low rate of rise of current (dl/dt). In addition, thyratrons cannot conduct large reverse current without damaging the anode. For those applications requiring simultaneous or precisely sequenced triggering of multiple switches, thyratrons are also inadequate because of the jitter in discharge ignition. Pseudospark switches provide solutions to many of these problems.
The conventional pseudospark switch was first reported by D. Bloess, et al., "The Triggered Pseudospark Chamber as a Fast Switch and as a High-Intensity Beam Source," Nuclear Instruments Methods, vol. 205, pp. 173-184 (1983), and a light activated version was taught by U.S. Pat. No. 4,771,168, to Gunderson. The authors describe a multigap "pseudospark" chamber for producing a controlled trigger mechanism for the fast switch.
The thyratron and "pseudospark" switches, generally operate in the low pressure regime where gas breakdown is limited by the distance between electron-gas particle collisions according to a law of electrophysics known as Paschen's law. Paschen's law defines the ability of gases to hold off a large voltage before "breakdown" and resuslting current flow as a function of the gas pressure and the spacing between electrodes. Paschen's law states that at high pressure, greater voltage standoff is achieved by moving the electrodes closer together. Experimental plots verifying Paschens law illustrate the region of operation of the pseudospark switch. FIG. 1 provides such an illustration, having the horizontal axis measured in the product of pressure times distance between anode and cathode or cathode and grid(pd) and the vertical axis measured in breakdown voltage. The Paschen curve bottoms out at about 7-13 mbar-mm, and as pressures are lowered below this range down to 10.sup.-3 Torr (a Torr equals approximately 1 mbar; 760 Torr equals one atmosphere pressure), the left side of the Paschen curve exhibits a sharp rise in the pressure-distance product. This means that to achieve greater voltage standoff in this region, the electrodes are moved closer together. This is region where the pseudospark switch operates. On this left side of the Paschen curve, by precisely lowering internal gas pressure within a gas filled tube, control of triggering may be maintained into the high voltage range. This is the parameter range in which a pseudospark switch operates. It is called a "pseudospark" because under these conditions discharge can be produced without collapse into a spark. Grid-controlled thyrotrons also operate on the left side of the Paschen curve in the lower valves of distance-pressure parameter.
A conventional round-hole pseudospark switch 10 has two metal plates 12 separated by an insulator 14 that is 1-3 mm thick (see FIG. 2). Each plate has a hole 16 (2-10 mm diameter) aligned with the hole in the opposite plate, with both holes coaxial with a similar hole in the insulator. Both plates and insulator operate inside a low-pressure housing (with pressures of several tenths of a Torr) containing gases such as hydrogen, nitrogen, helium, or argon. For commercial pseudospark devices, hydrogen is probably the best gas, thanks to the availability of low-leakage hydrogen reservoirs from the thyratron tube industry.
In round-hole pseudospark switches, the electrical breakdown voltage between a pair of parallel plates is a function of plate separation and gas pressure in the reservoir. For the pseudospark switch, the interelectrode gap is made to be about the same as the electron mean free path. Electrons then moving directly from electrode to electrode do not contribute to the ionization of the gas. A long path, however, is available the aligned apertures in the electrode plates to the back of the other electrode. This long path allows an electron to collide with the fill gas, resulting in a plasma. The switch discharge is triggered by generating or injecting electrons into the cathode plate with a laser, ultraviolet (UV) sources, flashlamps, or auxiliary electrode. The triggered electrons are accelerated through the hole in the cathode plate toward the anode plate, during which they collide with gas molecules to generate secondary electrons and ions. The secondary electrons travel toward the anode plate, generating additional collisions and secondary electrons. Meanwhile, the secondary-emission ions travel toward the cathode and impact it, generating electrons that in turn are swept toward the anode. As these high-energy electrons strike the anode, ions are released which travel back to collide with that cathode. These interactions generate a low-resistance plasma of electrons and ions, propagating through the aperture and connecting the backs of the electrode plates. This plasma conducts electricity, thereby producing closing-switch action. Because the plasma is diffuse, it does not erode the electrodes, ensuring long electrode lifetimes. Concurrently, the plasma discharge passage through the holes in the electrodes constricts it, increasing the plasma's temperature. In turn, this temperature rise lowers the resistance of the discharge, resulting in low switch losses.
Round-aperture pseudospark switches cannot be scaled to high power levels by increasing the radius of the aperture. Theoretical modeling indicates that increasing gap aperture reduces the switch's self-breakdown voltage (i.e., self-triggering) threshold. Lehr, J., et al., "The Linear Pseudospark: A High Current Pseudospark Switch," 1995 IEEE Pulsed Power Conference, Albuquerque, N. Mex. (July 1995). FIG. 3 shows the breakdown voltage as a function of hole radius for a conventional round hole pseudospark switch. Note that the breakdown potential goes to very low voltage as the hole radius increases for a specific pressure. Increasing the aperture makes the resulting discharge unstable and the switch ceases to function as a diffuse discharge and instead, collapses to an arc and then functions as a standard conventional spark gap with high electrode erosion and low voltage standoff.
Analysis and experiments of conventional round hole pseudospark switches indicate that the switch functions because of the peculiar field shaping produced by the roundness of the hole. A recent paper reviewing the state-of-the-art in pseudospark switches addressed the need for higher current capability. Frank, J., "Progress in Pseudospark Switch Development," 10th IEEE International Pulsed Power Conference, Albuquerque, N. Mex. (July 1995). The state-of-the-art according to Frank involves using several holes instead of a single hole to get an increase in current capability. Even with this configuration, Frank shows evidence of magnetic pinching, forcing all of the current to eventually flow through one hole which increases the erosion of the switch thereby reducing the lifetime. Bergmann has operated a version of the multiple-round aperture pseudospark switch with the holes arranged on the circumference of a circle so the switch current flows in the radial direction to reduce magnetic field pinching effects.
Meanwhile, several promising commercial applications of pulse power have emerged that highlight the performance and design limitations of current thyratrons switches and the demand for new switch technology. For example, high power particle beam accelerators require high low inductance, long current switches that are triggerable--requirements that are beyond the capabilities of thyratrons or spark gaps.
Thyratrons, moreover, due to the need for a physical grid, have a higher discharge losses; and the grid and cathode (even in cold cathode switches) experience degradation and limited life. Also, triggering by means of a physical grid interposed between the anode and cathode requires the use of an electrode trigger which is electrically coupled to the controlled high powered circuit. This electrical coupling of the controlled main high voltage circuit to the trigger necessarily introduces inherent safety problems.